The Topic of This Month Vol.18 No.12(No.214)

Influenza, 1996/97 Season, Japan

This topic concerns influenza during the last season (1996/97). Clinically diagnosed patients reported by pediatric and general physicians serving at about 2,500 sentinel clinics and isolation of influenza viruses as well as cases of severe complications reported by approximately 60 prefectural and municipal public health institutes in the whole country to the Infectious Disease Surveillance Center (IDSC), the National Institute of Infectious Diseases have been summarized.

Weekly cases of influenza in the past three seasons, 1994/95, 1995/96 and 1996/97, according to the National Epidemiological Surveillance of Infectious Diseases (NESID) are delineated in Fig. 1. In the last season, reports of cases started to increase from the 49th week (Dec. 1-7) 1996 and abruptly increased toward the 52nd week (Dec. 22-28), reaching the largest number in this period since influenza was recognized as a target disease of NESID in 1987. These patients did not develop into a large-scale epidemic, forming a peak in the fourth week (Jan. 19-25) 1997, and tended to decrease toward the 9th week (Feb. 23-Mar. 1). Being different from the usual trend, the number of patients was kept on a plateau in March and April in the last season, and then decreased gradually.

Fig. 2 shows the reports of weekly isolation of influenza viruses in the past three seasons in the same horizontal scale as in Fig. 1, and Table 1 the total reports of isolation by the subtype in the past 10 seasons. The reports on isolation of viruses to IDSC include that from specimens collected for investigation of the outbreaks occurring at kindergartens and primary and junior high schools in addition to that from specimens collected by sentinel clinics for NESID. In the 1994/95 season, type A (H3N2) was prevalent in the first half and so was type B admixed with a few type A (H1N1) viruses in the latter half of the season (Fig. 2 andTable 1). In the 1995/96 season, type A (H1N1) prevailing principally and type A (H3N2) joining to this were isolated in the latter half, but only a few type B viruses were isolated. In the 1996/97 season, mainly type A (H3N2) prevailed in the first half and type B in the latter half; type B was isolated continuously until July. Although the reports of isolation of influenza viruses in the last season totaled 5,861, which was the largest number ever reported, no type A (H1N1) virus was isolated. The type B virus isolated in Osaka Prefecture in February 1997 was a B/Victoria/2/87-like variant (see IASR, Vol. 18, No. 5). Isolation of similar type B variants was reported during April-June in Okayama and Hiroshima Prefectures (see IASR, Vol. 18, No. 6) and Kyoto City (see IASR, Vol. 18, No. 7). B /Victoria/2/87-like viruses were isolated in southern China in the 1995/96 season.

After consideration of the above results, four strains, A/Beijing/262/95 (H1N1), A/Wuhan/359/95 (H3N2), B/Mie/1/93, and B/Guangdong/05/94 (B/Victoria/2/87-like strain), are being utilized as the vaccine strains for the 1997/98 season in Japan (see IASR, Vol. 18, No. 10).

The age distribution of patients, from which viruses were isolated in the last season, shows that those yielding type A (H3N2) gave two peaks at 0-2 and 9-11 years (Fig. 3). Before December 1996, isolation was more frequent from elder children, and after January 1997, from younger children. Type B virus-yielding cases gave a peak at 6-8 years and the majority tended to be school children.

Severe complications of influenza are often central nervous system and cardiovascular disorders. Reports of patients of such severe complications from which influenza viruses were isolated have recently been increasing in the reports to IDSC (see IASR, Vol. 17, No. 11). In the last season, 10 cases of encephalopathy, seven of encephalitis, two of peri/myocarditis and one of Reye syndrome (Table 2), including one death and one brain death (see IASR, Vol. 18, No. 6), were reported. These reports included a case from which influenza virus genome was detected in the cerebrospinal fluid by RT-PCR. All these severe cases, except a case of unknown age, were children aged 0-12 years. Infants aged less than 2 years accounted for nearly half of them.

In aged patients, influenza tends to take severe courses; some cases, being complicated with pneumonia, may be fatal. It is known that there is epidemiologically the excess mortality* phenomenon during the influenza epidemic period. Little has been studied on influenza among aged people in this country, but in the last season, outbreaks of influenza at aged people's nursing homes in Okayama Prefecture, Hiroshima City and Tokyo Metropolis were reported to IASR (see IASR, Vol. 18, Nos. 5, 6 and 10). In the United States and European countries, giving influenza vaccination to the aged people over 65 years before the influenza season is emphatically being recommended (see p. 314 of this issue and CDC, MMWR, Vol. 46, RR-9). Vaccination of aged people on voluntary basis is available also in this country; detailed further investigation for the effects and safety is going to be carried out.

It was reported in May 1997 that influenza virus type A (H5N1) was isolated from the pharynx of a 3-year old boy died from pneumonia and Reye syndrome in Hong Kong. Type A (H5N1) is known as an avian influenza virus, and this was the first time to isolate it from a human source. An international cooperative study with member countries including Japan recognized no person-to-person transmission of the virus of this type around the patients (see IASR, Vol. 18, No. 9). It is 30 and 20 years, respectively, since the emergence of A (H3N2) and A (H1N1) viruses, now prevailing all over the world. Reassortance of genes occurs between avian or swine influenza viruses, and human influenza viruses and avian influenza viruses may directly invade humans. From these facts, isolation of A (H5N1) from a human source has been paid much attention from the whole world. In this country, a study group on emergence of new type influenza, having been organized by the Ministry of Health and Welfare, proposed a guideline to cope with the emergence and epidemics of new type influenza based on the principles of intensifying surveillance and vaccination as a counterplan (see p. 301 of this issue).

As of December 8, isolation of influenza virus has not been reported in this season.

*The increase in number of deaths when an influenza epidemic occurs from those on the assumption that no epidemic occurred.

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